The reprocessing of coated particle fuel e.g. from high temperature gas-cooled reactors (HTGR) is useful in many cases to maximize the use of the fuel. It is even mandatory for the U-Pu (fast reactor) and for the Th-U fuel cycle as well as for the incineration of minor actinides in these reactors.
Coated fuel particles are characterized by their high resistance against mechanical impact and chemical attack, which makes them safe, rather proliferation-resistant and suitable for direct disposal, yet difficult to reprocess.
For use in a reactor, the fuel particles are generally embedded in the matrix material (e.g. graphite, carbide or ceramic) of a fuel element. About 10 000 fuel particles are e.g. contained in a spherical fuel element (pebble) of type AVR GLE-4, fabricated by NUKEM for the German High Temperature Reactor (HTR). The following table summarises the nominal characteristics of AVR GLE-4 pebbles and the embedded particles.
Coated Fuel ParticleParticle batchHT 354-383Kernel compositionUO2Kernel diameter [μm]501Enrichment [U-235 wt. %]16.75Thickness of coatings [μm]:buffer92inner PyC38SiC33outer PyC41Particle diameter [μm]909PebbleHeavy metal loading [g/pebble]6.0U-235 contents [g/pebble]1.00 ± 1%Number of coated particles per pebble9560Volume packing fraction [%]6.2Defective SiC layers [U/Utot]7.8 × 10−6Matrix graphite gradeA3-3Matrix density [kg/m3]1750Temperature at final heat treatment [° C.]1900
For the reprocessing of coated fuel particles, i.e. the fuel kernel with multiple ceramic coatings, or fuel elements containing such fuel particles, one option is to isolate these particles from the matrix material (e.g. graphite) of the fuel elements, which may have the shape of spheres, rods, plates or other. Then the coatings of the fuel particles must be cracked to make the fuel kernel accessible to chemical reprocessing. Another option is to fragment the fuel elements and the coated particles together. A direct dissolution of the fuel element or of the coated particles is currently considered extremely difficult as in particular the often used SiC coating is resistant against dissolution in common nitric acid solutions. A mechanical cracking of the coatings is equally problematic due to the high mechanical resistance of the coatings and their questionable suitability for use in a hot cell environment.
In “An Overview of HTGR Fuel Cycle”, Report ORNL-TM-4747, 1976, K. J. Notz describes examples of mechanical fracturing applied to ceramic nuclear fuel, such as grinding by hammers (coal mill type) or grinding between disks (wheat mill type). U.S. Pat. No. 4,323,198 discloses the fragmentation by sandblasting against a hard surface for the reprocessing of nuclear particle fuel.
The known mechanical methods however suffer from several shortcomings:                low efficiency and high energy consumption;        potential use of pressurized gases as in the case of sandblasting;        production of toxic or explosive dust causing safety problems;        high noise level and vibrations;        pollution of the matter to be fragmented by non-negligible quantities of abrasion products;        high wear and tear of the impact material by abrasion, limited lifetime, high investment and operation costs.        
It can thus be concluded that mechanical methods are rather unsuited for use in hot cell environment as they are e.g. not compatible with the safety standards of the latter. High overall costs inhibits the industrial applicability of the mechanical methods.
In “The Reprocessing Issue for HTR Spent Fuels”, Proc ICAPP'04, Grenéche et al. discuss the pulsed high voltage discharge technique for separating coated particles from their graphite matrix. Experimental results show that graphite can be fragmented, and that the grain size distribution is a function of the number of applied pulses. The high voltage technique is known e.g. from Bluhm et al. “Application of HV Discharges to Material Fragmentation and Recycling”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 7 No. 5, 2000.
The cracking of coated fuel particles for reprocessing is, however, considered a so far unresolved problem.
Furthermore, when nuclear reactors of certain types (e.g. block-type reactors) are loaded, the fuel elements comprising the fuel particles are inserted into a support material of the reactor core. The support material depends on the reactor type and may e.g. comprise graphite, carbide or nitride. After operation, the extraction of the fuel elements and the radioactive material is difficult, because during operation of the nuclear reactor, the support material and the fuel elements may be subject to deformations. The fuel elements can be literally stuck in the surrounding support material.
Traditional methods for recovering the used nuclear fuel suffer from the disadvantage that the separation of the materials is not complete and that the support material remains contaminated with the nuclear fuel. Recycling of the support material thus is difficult.